Friction materials for use in brake systems for various vehicles, industrial machines, and so on are required to have high and stable coefficients of friction, excellent wear resistance, and low aggressiveness against rotors. To meet these characteristics, use was made as the friction materials of resin compositions in which asbestos, an inorganic filler, an organic filler, and so on are compounded with a thermosetting resin (hinder), such as a phenolic resin, for binding the above additives. However, because asbestos has been confirmed to be carcinogenic and is easily ground into dust, its use is refrained owing to environmental health problems attributable to its inhalation at work and potassium titanate fibers, metal fibers, and the like are used as alternatives.
Potassium titanate fibers have the advantage of keeping from damaging rotors. However, their wear resistance in a high temperature range is not adequate. As a solution to this, Patent Literature 1 proposes using as a friction modifier scaly magnesium potassium titanate (K0.2 to 0.7Mg0.4Ti1.6O3.7 to 4). Furthermore, Patent Literature 2 proposes using as a friction modifier scaly lithium potassium titanate (K0.5 to 0.7Li0.27Ti1.73O3.85 to 3.95).
Use is made as the metal fibers of copper fibers having low aggressiveness against rotors in order to enhance the strength of friction materials and improve the heat dissipation efficiency and wear resistance of the friction materials. To further improve the wear resistance, copper powder is also used in large amounts. However, as for friction materials containing copper, abrasion powder produced from them during braking contains copper, which leads to the suggestion that copper may cause river, lake, and marine pollutions and the like. Therefore, the amount of use of copper for friction materials has been restricted in North America. To cope with this, Patent Literature 3 proposes using as a friction material a resin composition containing lithium potassium titanate and graphite.